It is a convenient practice to physically separate a digital computer from the keyboard and display unit by which the computer is monitored and controlled. Actually the K.D.U. consists of two units, a cathode ray-type display and a keyboard. The cathode ray display or, as it is usually called, a monitor, and keyboard may be in the form of a single housed unit or, as more commonly today with microcomputers, the monitor and keyboard are separate. The most popular microcomputer presently in use is the IBM-PC microcomputer, and there are many units made by other manufacturers which generally employ the same or a similar arrangement for interconnecting a keyboard and monitor to a computer.
As a matter of convenience, there are separate electrical jacks on these computers for mating plugs, one plug connecting to a monitor cable and the other to a keyboard cable. The opposite ends of these cables have plugs which directly plug into a monitor equipped jack and a keyboard equipped jack. Normally these cables are on the order of four to six feet in length, enabling some, but limited, separation of a monitor-keyboard work station from a computer. This typically requires that the computer and work station be generally located together.
It is to be appreciated, however, that there are situations, in fact, many, where it is desirable to separate the computer and a work station. This may be by virtue of space limitations or because of environmental considerations, the latter sometimes including an inhospitable environment for a computer.
Separation has been attempted by extending the length of the separate connecting cables described above, but it has been found that where the separation is greater than, say, 12-16 feet, reliability is sacrificed. If a single cable housing both monitor and keyboard leads is attempted, as would be the ideal approach for greater distances, signal interaction becomes intolerable. Although it is believed many have attempted to solve the problems involved as they have been present for several years, insofar as applicants are aware, they have not succeeded and that no dual cable linkage system has been successfully employed beyond the 12-16 foot range, and no successful single ended TTL cable system has been accomplished other than by the applicants.
The applicants, and, it is believed, others, attempted to employ standard filtering techniques, as by adding capacitance between certain signal lines and ground. This simply does not work, and it appears that others simply gave up.
It is the object of this invention to provide an improved communications link by which a relatively long, single cable connects a computer with a keyboard and display unit and yet provides error-free communications.